Longitudinal changes in zooplankton distribution below a reservoir outfall with reference to river planktivory

The fate and interactions with river organisms of zooplankton as they drift downriver from a reservoir on a fourth-order mountain stream (Hiji River, Japan) were investigated. Monthly samples were collected at the reservoir and six river sites, simultaneously, from May 2005 to May 2006. Aquatic macroinvertebrates and fish were colleted, and their stomach contents were analyzed in April and May, 2006, respectively. Drift from the reservoir was the primary source for the river plankton community; the abundance of zooplankton, particularly those of cladocerans and large rotifer, rapidly decreased within several kilometers of the dam. Analysis of the contents of fish stomachs showed that drifting zooplankton was the main food for fish, with strong food selectivity for cladocerans and large rotifers. However, fish and insect planktivores showed longitudinally different stomach contents, with progressively fewer zooplankton found in the stomachs at the downriver sites. The results suggest that the outflow of zooplankton from the reservoir is an important food source for the downstream predators, especially fish, but the drift of zooplankton and consequent food availability for the predators at lower sites are strongly limited by concentrated fish predation just below the reservoir dam.     

A study of terrestrial and aerial macroinvertebrates on river banks and their contribution to drifting fauna and salmonid diets in a Scottish catchment

Macroinvertebrates of terrestrial and aerial origin were examined on the river banks, in the drifting community and in the stomach contents of salmonids at seven stream sites within the river Nethy catchment in Scotland over a 12 month period. Differences between sites (which varied from moorland to forest), had some influence on the taxonomic composition and diversity of riparian macroinvertebrates but not on the proportion of terrestrial and aerial organisms in the drift or in salmonid guts. The abundances of terrestrial and aerial organisms collected on the river banks and in the drift were not related to the combination of riparian vegetation and altitude. The abundance on the river banks of terrestrial and aerial macroinvertebrates, particularly those possessing wings, varied seasonally and was positively correlated with temperature. Terrestrial and aerial prey contributed up to 86% of the salmonid diet. Organisms most abundant on the river banks were not always the predominant organisms in the drift or those selected by salmonids. This study concludes that it is necessary to examine the riparian and benthic macroinvertebrates (the original prey sources) and the drift (as the actual source of salmonid forays) directly to elucidate salmonid trophic pathways.     

Short-term study testing the resilience of an estuarine bivalve to macroalgal mats

Increased mortality of juvenile Atlantic salmon (Salmo salar L.), related to lowered levels of stored energy following the loss of ice cover during winter, has been observed after hydropower development in the subarctic River Alta, northern Norway. Drift samples were compared to examine if drift densities, and thus drift prey availabilities for juvenile salmon, were lower in the ice-free than the ice-covered area. In addition, juvenile salmon stomach contents were compared to benthos and drift in the ice-free area to examine salmon winter feeding habitat. Zooplankton, originating from the reservoir, dominated drift at the ice-free site but had lower densities at the downstream ice-covered site. Excluding zooplankton, Chironomidae comprised most of the remaining drift at both the ice-free and ice-covered site, followed by Ephemeroptera, Plecoptera and Simuliidae. No Trichoptera were found in the drift samples. There was no consistent diel periodicity in drift. Benthos was dominated by Chironomidae, followed by Ephemeroptera, Plecoptera and Trichoptera. Other invertebrates occurred in low numbers. Juvenile salmon demonstrated size-selective feeding and fed mainly on Ephemeroptera, followed by Trichoptera and Plecoptera. No zooplankton and few Chironomidae were found in the stomach samples. Stomach content was more similar to benthos than to drift, indicating a larger extent of benthic than drift feeding. No evidence was found for the hypothesis that lack of ice cover reduced the invertebrate drift or caused diel periodicity in the drift. Differences in drift between areas with and without ice could not account for the observed differences in mortality of juvenile salmon during the winter in these areas.     

Feeding habits of horse mackerel, Trachurus trachurus (Linneaus, 1758), from the central Adriatic Sea

The feeding habits of the horse mackerel, Trachurus trachurus, from the central Adriatic Sea were investigated with respect to fish size and season. Stomach contents of 1200 specimens 12.9–37.6 cm total length (TL) taken at monthly intervals (January–December 1995) were analyzed. Of the total number of stomachs examined, 597 were empty (49.7%). This percentage varied significantly with season; the maximum number of empty stomachs was observed during January (80%) and February (82%) and a minimum number observed during July (15%) and August (19%). A total of 30 different prey species belonging to five major groups: Crustacea (Euphausiacea, Mysidacea, Decapoda), Cephalopoda, and Teleostei was identified in the stomach contents. Euphausiids were the most important ingested prey in all seasons as well as in small‐ and medium‐size classes (<28 cm TL). Two species of euphausiids, Nyctiphanes couchii (%IRI = 36.0) and Euphausia krohni (%IRI = 12.9), were the most frequent prey. The proportion of teleosts in stomach contents increased with the increasing size class of horse mackerel and was 15.5% IRI in >30 cm TL individuals. The mean weight of stomach contents increased significantly for fish larger than 24.1 cm TL, while the mean number of prey items significantly declined in the large‐size classes.     

Diet of the Worm Lizard,Diplometopon zarudnyi (Nikolsky, 1907), in Riyadh province,Saudi Arabia (Reptilia: Trogonophidae)

We analysed the diet composition of 133 specimens of the Worm Lizard, Diplometopon zarudnyi, in Riyadh province, Saudi Arabia. Analysis of stomach contents revealed that 90 specimens (66.6%) had prey items in their gut. The stomach content consisted mainly of small invertebrate prey (beetles), mainly larvae of Dermestes sp., but also imagines of Dermestes maculatus and Rhynchophorus ferrugineus. Dermestes sp. constitutes over 99% of the total food items, showing a high selectivity in food intake. The high number of beetles in the stomach contents indicates that this lizard forages very frequently. Specimens collected during winter had empty stomachs. The high proportion of fossorial prey items and the occasional records of other stomach contents show that D. zarudnyi only occasionally forages on the surface.     

Effects of alternative prey on predation intensity from herring Clupea harengus and sandeel Ammodytes marinus on capelin Mallotus villosus larvae in the Barents Sea

Stomach contents from lesser sandeel Ammodytes marinus and herring Clupea harengus caught at one and three stations, respectively, were analysed to investigate predation intensity on capelin Mallotus villosus larvae. Most capelin larvae were found in the anterior sections of the stomachs close to the oesophagus, indicating that rapid digestion rates of larvae reduced the abundance in the posterior sections. The anterior sections of the stomachs had alternating layers of either copepods or capelin larvae and krill. This shows that the individual predators switched between feeding on either copepods or on krill and capelin larvae. A total of 549 capelin larvae was found in 440 fish stomachs. Capelin larvae were found in 20% of the sandeel stomachs, and 24, 34 and 62% of the stomachs from the three herring stations, respectively. Many of the predator stomachs contained more than five larvae, and up to 28 larvae were found in one herring stomach. The herring appeared to prey selectively on the largest capelin larvae, and the number of larvae per stomach was much higher in stomachs where krill had recently been eaten than where copepods were dominant. The predation intensity measured in this study is much higher than reported in earlier studies, and it is suggested that the predators were feeding using a searching image, and that the concentrations of alternative prey (copepods and krill) varied around a switching threshold.     

Size characteristics and diet of emergent chinook salmon in a small, stable, New Zealand stream

The emergence timing, quality and diet of chinook salmon, Oncorhynchus tshawytscha , fry were studied as they emerged from ten natural redds. The impact of selected environmental factors on emergence was studied also. Samples of fry were obtained daily using redd traps.
The mean period of total emergence for redds with more than 450 fry emerging was 44.8 days (range 25–57 days), but for 80% of the fry in each redd to emerge the mean time elapsed was 10.8 days (range 6–21 days). No fry sampled had a visible yolk sac. The effect of temperature and light on emergence timing was weak, but stream flow had a strong influence. Fry length was constant for each redd throughout emergence, but fry weight tended to decline; consequently condition factor declined also. There was evidence that fry fed before emergence, but they did not grow. The major prey item, Deleatidium spp. (47.4% of the contents of all stomachs sampled), was the most abundant invertebrate in stream benthos and drift. Empty stomachs were prevalent at the start of emergence, with full stomachs becoming more common as emergence progressed.
The time distribution of numbers of fry emerging was usually unimodal, but occasionally bimodal. Over one third of fry emerged during the best 2 days of sampling.     

Molecular identification of prey in the stomach contents of Harp Seals (Pagophilus groenlandicus) using species-specific oligonucleotides

All methods of diet analysis in marine mammals, including hard part analysis (HPA), have biases affecting the accuracy of prey-species identification and frequency in the estimated diet due to differential consumption, digestion and retention. Using PCR amplification of specific prey DNA with species-specific primers, we developed a DNA-based method that complements HPA and provides an alternative means to detect prey from stomach contents of Harp Seals (Pagophilus groenlandicus). The target size that could be reliably amplified was determined using a digestion time-series of Atlantic Cod (Gadus morhua) tissue in simulated seal stomachs. Various target lengths were trialed using general teleost primers; amplicons of approximately 800 bp or less were consistently obtained. Prey species-specific PCR primers for Atlantic Cod, Arctic Cod (Boreogadus saida) and Capelin (Mallotus villosus) were designed and tested with DNA from the stomach contents of 31 Harp Seals. Amplicons were obtained for all three species-specific primer sets. Amplification results compared with HPA revealed: (i) Atlantic Cod hard parts were found in five stomachs where no Atlantic Cod DNA amplified, suggesting that Atlantic Cod may be over-represented in the estimated diet, (ii) amplification of Arctic Cod DNA occurred for 17 stomachs, including all 12 stomachs with, and five stomachs without, Arctic Cod hard parts, and (iii) Capelin DNA amplified for four of five stomachs with Capelin hard parts and for one stomach without Capelin hard parts. We conclude that PCR amplification of specific prey DNA provides a viable means to complement Harp Seal diet analysis by HPA, but suggest that valuable information for quantitative diet analysis rests in a quantitative PCR approach.     

渤海鱼类食物关系的初步研究

本文材料取自1982年4月—1983年5月在渤海逐月进行的拖网探捕调查。共分析了54种鱼的1,863个胃。用3个指标确定各种饵料在鱼类食谱中的重要性:(1)重量百分比(W);(2)尾数百分比(N);(3)出现频率(F)。根据3个指标计算每种饵料的相对重要性指标:IRI=(N+W)·F。按照IRI的大小,渤海的鱼类大体上可分成5个类群:(1)浮游动物食性;(2)浮游和底栖动物食性;(3)底栖动物食性;(4)底栖和游泳动物食性;(5)游泳动物食性。这些龟类按Odum和Heald的方法可分成3个营养级:(1)低级肉食性鱼类;(2)中级肉食性鱼类;(3)高级肉食性鱼类。 在渤海饵料生物中,起主要作用的是浮游动物、鼓虾、短尾类、尖尾鰕虎鱼、钝尖尾鰕虎鱼和(鱼是)鱼.根据主耍饵料,渤海鱼类可以分成5个类群:(1)以黄盖鲽为代表,它们主食腔肠动物、多毛类和棘皮动物;(2)以鳎类为代表,主食鼓虾和短尾类;(3)以鳐类、石首鱼类为代表,主食鼓虾和钝尖尾锻虎鱼;(4)以(鱼是)鱼为代表,主食浮游动物;(5)以牙鲆为代表,主食鱼类。5个类群之间,仅第2、3两个类群在鼓虾上有食物竞争。 在除掉渤海湾、辽东湾及5米以内浅水区外的调查海区内的54种鱼1863个消化道中,仅发现8尾对虾,它表明对虾几乎没有被其它鱼类捕食。对近岸及各湾的幼体和仔、幼虾来说,可能并非如此,需要进行专门的调查后才能弄清。     

Notes on New Zealand mammals 6. Second report on the stomach contents of long‐finned pilot whales,Globicephala melas

Abstract

We report data on the stomach contents of the long‐finned pilot whale, Globicephala melas, recovered from a group of whales stranded on Ruakaka Beach, northeastern New Zealand, in November 2006. In nine whales for which identifiable stomach contents were recovered (three that stranded on 10 November and six that stranded on 11 November) prey remains comprised exclusively cephalopod beaks attributed to five squid species. The stomachs of a further two whales contained unidentifiable upper beaks only, while the stomachs of five whales were completely empty. No whale appeared to have been satiated immediately before stranding, given that the maximum biomass of prey recently consumed by any one whale was calculated to be <5 kg. All squids ingested represented oceanic species, found from 50 to 1000 m but more common towards the deeper end of this range. These data both complement and contrast with the only other dietary information available for this species in New Zealand waters, reported from stomach contents of whales stranded on Farewell Spit, South Island in December 2005.     

Prey selection and diel feeding of the freshwater jellyfish, Craspedacusta sowerbyi

1. The diet of the invasive freshwater jellyfish, Craspedacusta sowerbyi (Lankester), was assessed by analysing its stomach contents. 2. The medusae ingested various zooplankton and benthic prey in the 0.1–3.0 mm size range. The selectivity indices for prey showed that larger zooplankton (0.4–1.4 mm) and active prey such as copepods were preferred; small potential prey, and the loricate rotifer Keratella cochlearis in particular, were almost never consumed. Measurements suggest that spacing between the tentacle branches could determine prey selection. Another possibility is that some prey are too small or slow to activate nematocysts after collision with the predator. Prey > 1.4 mm can probably escape or are otherwise too large to be handled. 3. The medusae of Craspedacusta medusae stayed in deep water during daytime and migrated upwards at night. The greatest number of prey in freshwater jellyfish stomachs was detected at night. 4. Calculation of consumption rates, based on mean stomach contents and digestion times of 4–5 h, showed that, on average, one medusa takes 190 zooplankton prey day^??1 at a mean prey density of about 100 L^??1. At the observed medusae density of 1 m^??3, the cropping rate was about 0.2 zooplankton prey L^??1 day^??1.     

Effects of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM) on the macroinvertebrates of Wilmot Creek

The effects of the lampricide, TFM, on the abundance of macroinvertebrates in the benthos of Wilmot Creek, a hardwater tributary to Lake Ontario, was examined over 1 year. Drifting macroinvertebrates were also collected before, during and after TFM treatment. Significant decreases in benthic abundance were exhibited by Dolophilodes sp., Tubificoidea, Cricotopus sp. and Macrotendipes sp. throughout the 350 days following treatment. Only the decrease in abundance of Dolophilodes sp. and Tubificoidea could be attributed to TFM treatment. Increases in drift abundance observed during treatment were generally an accurate indicator of TFM-sensitive macroinvertebrates. The most sentive taxa (Dolophilodes sp., Dugesia sp. and Tubificoidea) responded immediately following the introduction of TFM. Branchiobdellida, Diamesa sp., Dicranota sp., Lumbricidae and Nemouridae exhibited increases in drift abundance 8–10 h after the introduction of TFM, however, were considered less sensitive than the former taxa because a decline in their abundance in the benthos was not detected.The response of the benthic invertebrates found in this hardwater creek was similar to those observed during studies of softwater streams. Only the most severely affected taxa were not present in the benthos 350 days after treatment.     

Feeding and prey-selection of wild Atlantic salmon post-smolts

The diet of post-smolt Atlantic salmon Salmo salar caught in the Trondheimsfjord and Frohavet in central Norway, based on stomach contents analysis, showed a gradual change during migration from the river to the estuary, fjord and coastal areas. Post-smolts caught in the estuary had eaten intertidal gammarid amphipods, while post-smolts caught further seawards preyed upon available marine prey such as Calanus spp., adult euphausiids and fish larvae. The frequency of adult insects was high in all post-smolt stomachs. The gradual change in diet suggested that feeding conditions in the early marine phase were important for post-smolt survival and growth. With the exception of the copepods, there was no overall similarity between species composition of the plankton samples and the stomach contents. Although the hypothesis that the post-smolts are opportunistic feeders cannot be rejected, the composition of the stomach contents suggests a possible selectivity of advantageous prey.     

Diet and foraging of the endemic lizard Cnemidophorus littoralis (Squamata, Teiidae) in the restinga de Jurubatiba, Macaé, RJ.

We investigated the diet and foraging of the endemic teiid lizard Cnemidophorus littoralis in a restinga habitat in Jurubatiba, Macaé - RJ. The stomach contents were removed, analyzed and identified to the Order level. There was no relationship between C. littoralis morphological variables and number, length or volume of preys. Termites (48.7%) and larvae (35.5%) were the most important prey items which occurred in the examined lizards' stomachs. The diet did not differ between males and females. Cnemidophorus littoralis is an active forager and predominantly consumes relatively sedentary prey or prey that is aggregated in the environment. We also found an intact and undigested hatchling of the crepuscular/nocturnal gekkonid lizard Hemidactylus mabouia in the stomach of an adult male of C. littoralis, which indicates that C. littoralis is a potential source of mortality for individuals of H. mabouia in the restinga de Jurubatiba.     

Long‐term changes in the feeding of Pollachius virens on the Scotian Shelf: responses to a dynamic ecosystem

The diet and feeding ecology of pollock Pollachius virens from the Scotian Shelf and Bay of Fundy in the north‐west Atlantic changed over the last few decades, which was associated with decreases in euphausiid abundance. Stomach contents data for 2078 pollock collected during the 1958–1967 period and for 1230 pollock collected during the 1996–2002 period indicated that pollock diet contained fewer euphausiids and feeding activity decreased. During the early period, euphausiids were present in 65% of the pollock stomachs that contained food and only 9% of these stomachs in the recent period. The decrease of euphausiids was not wholly offset by an increase in piscivory, since there was little increase in the frequency of fish prey in the diet or in the fullness index for this prey type. Empty stomachs were significantly more common in the recent period during both the winter and summer. The decreased occurrence of euphausiids in stomach samples coincided with a significantly decreased abundance of this prey, suggesting that the near‐absence of euphausiids in recently collected pollock stomachs reflected prey abundance. Concurrent with changes in diet and feeding intensity, the condition or 'plumpness' of pollock significantly declined from the early to the late sampling periods.     

Benthic, Drifting and Marginal Macroinvertebrate Assemblages in a Lowland River: Temporal and Spatial Variations and Size Structure

Aquatic macroinvertebrates living in anastomosing lowland rivers use different habitats and respond differently to the hydrological regime. In this paper, the structure and composition of benthic, drifting and marginal macroinvertebrate assemblages are analyzed in the lowland river Ctalamochita (Córdoba, Argentina). The assemblages were studied in an annual cycle; a comparison among the composition of benthos, drift and marginal fauna was carried out; and size structure of the assemblages was characterized. Samples were obtained from two sites: a rural and an urban site. In total 73 taxa of aquatic macroinvertebrates were collected. Benthos was characterized by Chironomidae and Oligochaeta; marginal fauna was mainly constituted by Coleoptera, Heteroptera, Decapoda, the Trichoptera Nectopsyche sp., Ephemeroptera and Odonata. Drifting assemblage was composed by macroinvertebrates from local and remote upstream benthos, and from the marginal zone. Marginal fauna diversity was higher than benthos and drift. Total biomass of the assemblages pooled together was relatively equitably among size classes. Larger size classes consisted of organisms from the marginal zone whereas the smallest ones were composed by benthic and drifting organisms. In the study area there is habitat partitioning in the lateral dimension of the river. Marginal fauna was more diverse due to the asymmetry of transport and deposit processes, which generate a heterogeneous habitat in the bankside. The relation between fine substrate and high current velocity determines an unstable habitat in the central channel, which makes colonization by benthic macroinvertebrates difficult.     

Fish eggs and larvae in the diet of herring Clupea harengus membras Linnaeus, 1758 and the sprat Sprattus sprattus balticus (Schneider, 1904) (Clupeidae) in the Southeastern Baltic Sea

The qualitative and quantitative composition of fish eggs and larvae in the stomach contents of Baltic herring and sprat was examined. The occurrence of fish eggs and larvae in the diet of herring was markedly higher, compared to sprat. Eggs of four and larvae of nine fish species were found in the herring stomachs. The prey species composition of the herring and sprat diets was related to the ichthyoplankton composition and to the degree of overlapping of the prey and predator distributions. Sprat eggs were the predominant prey in the herring diet in May and sprat larvae prevailed in August. The daily ration estimates indicate that the sprat larvae consumption by herring in August exceeded the sprat egg consumption in May by many times. Cod eggs in the herring stomachs were only found at the optimal oxygen content and salinity for the cod eggs in the near-bottom water layer.     

Prey selection by young trout fry (Salmo trutta)

Factors influencing the choice of invertebrate food by early feeding trout are sought, special attention being given to the dimensions of the prey. The stomach contents of fish of similar age feeding on lake plankton and drifting stream fauna are compared. Little consistent preference was displayed for the smaller planktonic organisms but prey size was of importance in the distribution of stream benthos among predators of different lengths. The width of the prey was of greater significance to selection than the mean volume or the area of the largest surface of the food items. When the fish were in obvious competition for food the correlation between predator length and all prey dimensions improved.     

洗胃法与剖胃法在四种蛙食性分析中的对比

在动物的食性研究中,剖胃法因要杀死动物,会产生一系列的生态学影响和伦理学问题,在国际上招致了越来越多的反对,而在我国仍广泛使用。尽管洗胃法已用于食性研究,它的效果却从来没有研究过。我们应用洗胃法研究了浙江宁波郭巨镇四种蛙(黑斑蛙、泽蛙、金线蛙和中华蟾蜍)的食性,并与剖胃法所获得的食性结果进行比较。结果显示:尽管洗胃法在四种蛙胃内食物量的总洗出率(在94.50%以上)和食物个体数的洗出率上(均大于93.46%)略低于剖胃法,但胃内未洗出的与洗出的平均食物个体大小间无差异(所有P>0.3061),两种方法获得的四种蛙的平均食物量和食物个体数也无差异(对食物量所有P>0.8680,对食物个体数所有P>0.7923)。结果表明,洗胃法是比较精确的,在动物的食性研究中,应广泛地推广洗胃法,摒弃剖胃法     

Intra- and interspecific dominance hierarchies and variation in foraging tactics of two species of stream-dwelling chars

Aggressive interactions, foraging behavior, habitat use and diet were studied in sympatric populations of white-sported char,Salvelinus leucomaenis, and Dolly Varden,Salvelinus malma, in a Japanese mountain stream. Underwater observations on individuals of both species revealed two distinct behavioral regimes: aggressive drift foragers and non-aggressive benthos foragers. Aggressive drift foragers defended partial territories around focal points from which they made forays to capture invertebrates drifting in the water column. Non-aggressive benthos foragers cruised around and beneath cobble in large foraging ranges that overlapped each other. Intra- and interspecific, size-dependent dominance hierarchies were recognized among aggressive drift foragers, whereas non-aggressive benthos foragers showed no such relationships. Terrestrial invertebrates were the most abundant prey in the diets of drift foragers, whereas a very small proportion of the diet of benthos foragers was made up of these taxa. Benthos foragers showed more complex diet composition than drift foragers. These results suggest that non-aggressive benthos foragers may avoid not only interference but also exploitative competition by using alternative foraging tactics. The proportion of drift foragers to benthos foragers among white-spotted char was more than 35 times that among Dolly Varden. The significant difference in the proportion of each species using the two types of foraging strategy results in interspecific food segregation in sympatric populations.